E: Engine Crankshaft Balance Method

[Albervin]

I have just read an article in 40 years on by Jacqueline Bickerstaff. She talks about vibration moments. How they vary according to frame and cylinder orientation. I think people need to get a handle on the fact that without a contra rotating balance shaft there will always be vibration somewhere. Personally, I would want that vibration at 120 mph because I will never be there. At the moment I have a Rapide and a Shadow that vibrate at 70mph. That really, really, annoys me. I am now going to put a 46T rear sprocket back on the Rapide so the vibration hits at closer to 80mph. BUT do I increase or decrease the balance on the shadow so vibration comes in at either 30 mph or 110 mph? Basically I want 50-80mph smooth in top or third. Over to you.

 

[Vincent Brake]

Alyn, i have very good results in over 50%
B is at 52, C at 50 and BS at 53%

they shake a bit their tail at a traffic light, but dead smooth at 80mph,
at 90 they shake a bit.
i ride them 22 front 47 rear sprocket and all with intermidiate gearbox, nice with all those roundabouts here.

47% as in early days is really tooooo less


over to down under

 

[Bill Thomas]

It might be the crank is not as true as was when put together.
When I first built my L/ning it was super, But after the first race, Some vib's were there, Not too bad.
Years later as a road Bike, I was not happy, Big Sid told me to put loose lead weights/balls, Between corks,
Maybe 5" between corks one side and maybe 4" the other,
Works on my Bike and Cheap !, 70 mph now smooth. Cheers Bill.

 

[Albervin]

It might be the crank is not as true as was when put together.
When I first built my L/ning it was super, But after the first race, Some vib's were there, Not too bad.
Years later as a road Bike, I was not happy, Big Sid told me to put loose lead weights/balls, Between corks,
Maybe 5" between corks one side and maybe 4" the other,
Works on my Bike and Cheap !, 70 mph now smooth. Cheers Bill.

"Corks"? Do you mean Forks Bill?

 

[Albervin]

OK. So that is about speed. What about vibration on acceleration? I have become used to changing vibration frequency when accelerating then roll off and all smooths out. I am wondering now if my flywheels are out. Really, I should just ride and not read.

 

[Bill Thomas]

Corks as in wine bottles, The balls should be able to move about, But not too much,
The inner corks stops the balls going along the handle bars.
So 4 corks with different gaps. Worth a go ?,I got my balls from a fishing shop.
I didn't think you could buy lead balls for fishing, But I had no trouble.

 

[Nigel Spaxman]

My bike didn't vibrate at all until I added the luggage rack. It is 46% dynamic balanced.

 

[Monkeypants]

Remove that luggage rack and get a Kriega backpack for Allyson.
Tell her it's for the greater good!

Glen

 

[oexing]

Regarding dynamic balancing, I got another two SR 500 cranks for balancing and tried to do videos about this action. I have just uploaded them into youtube, see link below. Please read the description under the video there for more info.
Amazing how far off these cranks are because of forging or casting processes. I did not dare to go over 1000 rpm at first try for fear of having the crank vacate my test rig with vibrations. I am still in learning mode with the electronics and want to see what low limits I can achieve . When aiming for under one gramm unbalance I spend quite some time on this as angular positions are only by eye and hand drill machining on the test rig itself. So I start with adding small magnets that I can shift to see what that does to results. Finally there are stacks of magnets to show me what amount of steel I got to drill out at the opposite positions for the real balancing. Yes, the test electronics like lots of inputs about diameter of crank webs, if radial drilling or axial, what size of drill , what sort of metal is to be drilled, andso on. So I basically get all info for quick balancing - but in real life you sometimes do not want to drill exactly in proposed positions on a crank web in any case so you go for compromises and some - or more - trial and error till you are happy with the job. To be open, I could easily much earlier declare the job done and fit for purpose really. But this is some sports for me to see fractions of one gram finally for satisfaction.
In case of Vincent cranks I don´t think you´d feel a dramatic difference after electronic balancing as all parts of the crank were machined to decent tolerances I guess, but then you cannot know. So a rough engine may have other defects, wrong balance factor one of these.

Vic
Edit: Sorry, seems I uploaded one clip twice in youtube, so you may want to go faster forward a bit.

SR 500 crank dynamic balancing

 

[greg brillus]

I think you'll find that if all the parts are in top order (especially new stuff) that dynamic balancing offers very little advantage over doing it manually with a set of knife edges, some decent scales and a bit of time and patience. Setting the balance factor is one thing, but whether it works in that particular engine/frame combination is another.........I'm about to do 2 Vincent cranks as we speak, one is the same as the one Dave is trying to set up with Carillo rods, the other with stock Vin rods.